Select mouse embryo cells, and cells of other species of interest with mutations which render the cells unable to grow at a certain temperature, but still capable of growth at a lesser temperature. Select from the temperature-sensitive mutants, clones with defects in specific biochemical pathways. Cells will be selected initially for defects in DNA or RNA synthesis. Characterize the mutant clones with respect to the specific genetic lesion. Propagate cells at every stage of selection, maintain frozen samples, and make these samples available for distribution as directed by the Project Officer.